The present invention relates to a fuel pump for an internal-combustion engine. In particular, the present invention relates to a fuel pump for a system for direct injection of petrol, to which the ensuing treatment will make explicit reference without this implying any loss of generality.
In systems for direct injection of petrol, it is known to use a petrol pump, which comprises a pumping chamber delimited at the bottom by a mobile surface so as enable cyclic variation of its volume. Set in a position corresponding to a top surface of the pumping chamber is a one-way intake valve, which sets the pumping chamber in communication with a petrol tank to enable a flow of petrol towards the pumping chamber, and a one-way delivery valve, which sets the pumping chamber in communication with a pipe for supplying the injectors to enable a flow of petrol under pressure from the pumping chamber.
Generally, both the intake valve and the delivery valve are of the ball type, i.e., each valve comprises a spherical valve body, which is mobile, along a channel for supply of the petrol, and a valve seat, which is designed to be engaged in a fluid-tight way by the spherical valve body and is set at one end of the supply channel. A spring pushes the spherical valve body towards a position of engagement of the valve seat in a fluid-tight way.
A ball valve has quite a high inertia due basically to the non-negligible mass of the spherical valve body and to the need to overcome during opening the elastic force generated by the spring. The high inertia of a ball valve is negligible when the difference of pressure across the valve is high, but becomes significant when the difference of pressure across the valve is reduced. Consequently, a ball valve is suited for being used as delivery valve of a petrol pump, but proves less suited for being used as intake valve of a petrol pump in so far as it is relatively slow.
For example, a petrol pump with a single pumping element (i.e., with just one compression chamber) can arrive at performing 14 000 pumping strokes per minute when the motor is at the maximum r.p.m., namely 7 000 r.p.m. Consequently, it has been estimated that in a four-cylinder engine with approximately two litres of displacement, the mean capacity of the pump, and hence of each valve, can be in the region of 150 litres per hour, whilst the instantaneous maximum capacity of the pump, and hence of each valve, can arrive at 800 litres per hour. To reach an instantaneous capacity of 800 litres per hour, carrying out 14 000 pumping strokes per minute, it is necessary for both of the valves to be particularly fast both during opening and during closing.
U.S. Pat. No. 6,223,724 disclose a high-pressure fuel pump, in which an opening of the low pressure fuel inlet and a high pressure fuel outlet are controlled by a valve system comprising a pair of plates and a flexible sheet having flaps of larger dimensions than the inlet dimensions but smaller than the outlets.